pipe: added music_buffer, rewrite music_pipe

Turn the music_pipe into a simple music_chunk queue.  The music_chunk
allocation code is moved to music_buffer, and is now managed with a
linked list instead of a ring buffer.  Two separate music_pipe objects
are used by the decoder for the "current" and the "next" song, which
greatly simplifies the cross-fading code.

src/decoder_control.h

@@ -66,6 +66,12 @@ struct decoder_control {
 	struct song *current_song;
 	struct song *next_song;
 	float total_time;
+
+	/** the #music_chunk allocator */
+	struct music_buffer *buffer;
+
+	/** the destination pipe for decoded chunks */
+	struct music_pipe *pipe;
 };
 
 extern struct decoder_control dc;